Control systems of brush DC motors use information about the current rotation speed of the motor.
Commonly used detection methods of the rotation speed are based on the use of Hall sensors that count the number of passages of a magnet mounted on the shaft, or of an encoder mounted on the shaft that establishes the angular position of the shaft. These systems use 3 connection lines for each sensor mounted on the motor, and thus additional cabling besides the cabling to power the motor.
Alternative methods for sensing the angular position of the rotor without using sensors, based on the analysis of the armature current for detecting the ripple induced by brush switchings, have been developed. FIG. 1 illustrates the detection principle. The origin of the switching ripple on the armature current of a DC motor is well known to a skilled person and it is considered superfluous to discuss it in this document.
A technical challenge exists in counting the peaks of the armature current, the waveform of which, besides the ripple due to the switching of the brushes, has numerous and different irregularities because of wear and tear of the motor, physical phenomena tied to the functioning of the motor, and disturbances injected in the power line. In practice, the noise that is typically present has a frequency and an amplitude spectra similar to that of the useful signal. Moreover, the strong variability of the parameters that may cause spurious effects on the armature current signal is such to generate ripple (peaks) also when the useful signal is momentarily missing, and this increases relevantly criticalness of the control.
Some of the main causes of disturbances that an appropriate filter would be capable of discriminating include that the rotor magnetic field generates a BEMF outphased in respect to the BEMF due to the variation of the stator magnetic field, as illustrated in FIG. 2. This phenomenon generates a double waveform during switchings of the brushes, as illustrated in the diagrams A) and B) of FIG. 3, relative to two functioning conditions of a DC motor for car window movements. The Fourier analysis illustrated in FIG. 4 shows that the main component of the ripple frequency may be totally missing. From this it is possible to infer that a filter based on frequency analysis could be inadequate.
Another cause may be that the current signal is missing for a significant time interval. Typically this phenomenon occurs during the switching of the driving relays that may last longer than 10 ms, as shown in FIG. 5. During this time, up to five peaks may be lost, corresponding to more than half a turn of the motor.
Still another cause may be that the enabling of the motor generates a double peak due to the start-up current, as shown in FIG. 6.
A further cause may be that current peaks not due to switching of the brushes are generated by noise injected through the supply line. If the frequency of the motor is comparable with the switching frequency of the brushes, peaks generated by noise could be detected and generate a false counting.
Yet another cause may be that brush motors for the car industry have generally a reduced number of brushes and thus the counting must be precise for satisfying the required standards of automotive systems. It is thus helpful to reduce the counting error due to the reduced number of brush switchings, with a system adapted to assess intermediate positions of the rotor between two brush switchings. Moreover, the fact that the intermediate position is not known may generate counting errors because it is difficult to synchronize the filter signal with the signal to be analyzed from the first pulse.
Among the relevant prior art documents about processing of the armature current signal in order to minimize the estimation error of the angular position and the rotation speed of the motor, the following may be mentioned:
EP-A-0 890 841-B1 (TEMIC) compares the state equation of the motor for estimating the ripple frequency and compares the measured ripple frequency of the current with the estimated frequency.
EP-A-0 689 054-B1 (BOSCH) discloses a totally analog circuit with amplifier of a current signal, analog filter that carries out a phase shift, filter and comparator that “squares” the ripple deleting noise.
U.S. Pat. No. 6,859,030 (KOSTAL) discloses the detection of the ripple during a brake phase for estimating the shift while braking.
U.S. Pat. No. 6,768,282 (KOSTAL) discloses the step of comparing the time between two peaks with an estimated time and thus delete a peak if the difference surpasses a certain threshold.
U.S. Pat. No. 7,079,964 (KOSTAL), discloses a spectral analysis of the current signal and of the applied voltage signal. The comparison between the two spectral analysis allows to discriminate frequencies introduced by the current ripple of the motor from the frequency due to noise on the supply line.
U.S. Pat. No. 6,839,653 (KOSTAL) discloses the Fourier analysis of the current signal for discriminating frequencies due to the current ripple from frequencies due to noise.
EP-A-1 453 172-A1 (LEAR) discloses the analysis of the motor current and the use of the current ripple to detect the rotation of the motor shaft. The contemplated filters are relatively complex and are not simple to be realized.